1. Field of the Invention
The invention relates to a thin-film magnetic field sensor.
2. Discussion of the Background
The production of a reliable magnetic sensor in the 10.sup.-6 -10.sup.-1 oersted magnetic field range remains a strategic objective, both for the detection of magnetic anomalies and for producing read heads for reading high-density magnetic storage media.
Among the physical effects that are sensitive to the magnetic field, magneto-transport phenomena have the advantage of being able to be easily integrated into the read electronics. At the present time, two types of effect are used: on the one hand, the magneto-resistance of magnetic materials and, on the other hand, the Hall effect.
French Patent Application No. 93/15551 describes a sensor for detecting a low magnetic field, based on a planar photosensitive Hall-effect element. This patent application may serve as the basis of the closest prior art. U.S. Pat. No. 4,987,509 describes a system comprising two magnetoresistive elements placed so as to be parallel to each other and a conductor placed between these two elements in order to induce a polarization field and to determine, when not in use, a direction of the magnetization in each element.
Such a sensor relies on the transverse measurement of the anisotropic magnetoresistance effect in a thin ferromagnetic film. FIG. 1 shows the magneto-sensitive layer. A current I flows in a direction XX' of the layer and the resistivity is measured in the direction YY'. This resistivity varies with the magnetization M applied to the sensor. In addition, it depends on the angle .theta. between the magnetization and the direction XX' of the current: ##EQU1##
The voltage measured in the YY' direction varies with the angle .theta. between the magnetization of the film and the measurement current, according to the equation below: ##EQU2##
where .DELTA.R is the resistivity anisotropy, which depends essentially on the material and on the thickness of the active part of the sensor.
The two main advantages of planar Hall-effect sensors over longitudinal-measurement magnetoresistive sensors are, on the one hand, a great simplification in the associated technology and, on the other hand, a reduction by approximately four orders of magnitude in the thermal drift, the main noise component at low frequencies (around 1 Hz). By construction, this sensor may be made sensitive only to the magnetic field component perpendicular to its direction of supply. Its size may be reduced to dimensions smaller than those of the magnetic domains, thereby eliminating the source of noise associated with the movements of the walls. Measurements on prototypes of such sensors have shown that these sensors have a linear response over four orders of magnitude (see document A. Schuhl, F. Nguyen-Van-Dau and J. R. Childress, Applied Physics Letters, 66, May 15, 1995).
However, in cases in which the two easy axes of magnetization of the magnetoresistive layer of the sensor are equivalent (that is to say have equivalent values) a problem arises in determining the situation when the sensor is not in use.